[Garage Mode] Back-port null checks from master

Add null checks when counting how many idle-mode jobs are running.

Bug: 146671528
Test: On Hawk
Merged-In: If2e22599c62deffa364043f4e6d4ce86fd69b8a2
Change-Id: If2e22599c62deffa364043f4e6d4ce86fd69b8a2
diff --git a/service/src/com/android/car/garagemode/GarageMode.java b/service/src/com/android/car/garagemode/GarageMode.java
index 4be06ff..35a9809 100644
--- a/service/src/com/android/car/garagemode/GarageMode.java
+++ b/service/src/com/android/car/garagemode/GarageMode.java
@@ -232,7 +232,7 @@
         int count = 0;
         List<String> currentPendingJobs = new ArrayList<>();
         final List<JobSnapshot> allJobs = mJobScheduler.getAllJobSnapshots();
-        if (allJobs != null) {
+        if (allJobs != null && startedJobs != null) {
             for (JobSnapshot snap : allJobs) {
                 if (startedJobs.contains(snap.getJobInfo())
                         && snap.getJobInfo().isRequireDeviceIdle()) {